1. Field
This invention relates to an image synthesis apparatus and an image synthesis method.
2. Description of the Related Art
There has been an image synthesis apparatus that synthesizes images around a vehicle. The images are taken by a plurality of in-vehicle cameras mounted on the periphery of the vehicle, and the synthetic image is displayed, thereby facilitating the grasp of the situation around the vehicle and the confirmation of safety by a driver.
For example, as illustrated in FIG. 19, in-vehicle cameras C1 to C4 are respectively provided on the front, back, left, and right sides of a vehicle, and an image in each direction is taken by the in-vehicle cameras C1 to C4. Image processing, such as viewpoint conversion, is applied to the taken images that are to be synthesized, allowing the synthesized images to be displayed as one image, as shown in FIG. 19, as if the driver's own vehicle were viewed from above. Such a synthetic image is generated and displayed by the prior art image synthesis apparatus, allowing a driver to grasp the situation around the driver's own vehicle by one image. In addition, the positional relation between the driver's own vehicle and obstacles can be easily confirmed.
However, when images are simply synthesized in the above apparatus, for example, automatic adjustment may be performed due to a difference in the direction to a light source, so that the cameras C1 to C4 have different irises and so on. Especially, in image taking regions E1 to E4 of the cameras C1 to C4, when a brightness value of regions R1 to R4 (hereinafter referred to as “overlap regions R1 to R4”) overlapping the image taking region E of the adjacent camera C is different from a brightness value of the overlap region R of the relevant adjacent camera C, there is a problem that joints b1 to b4 between each image are conspicuously displayed in the generated synthetic image.
In order to solve the above problem, a technique has been developed for correcting a pixel value of a taken image by image processing to thereby generate a natural-looking synthetic image.
Japanese Laid-open Patent Publication No. 2008-77595 discloses calculating correction factors so that the brightness value of the overlap regions R1 and R2 in the camera C1 and the brightness value of the overlap region R1 in the camera C4 and the overlap region R2 in the camera C2 are equal to each other. The average value of the calculated correction factors of the overlap regions R1 and R2 is mapped to the correction factor of the entire image taking region E1 of the camera C1.
For example, when an image taken by the camera C1 is corrected, the correction factor in a pixel h1 in the overlap region R1 is represented as G(h1), and the correction factor in a pixel h2 in the overlap region R2 is represented as G(h2). As illustrated in FIG. 20, the average value G(h) of the correction factors G(h1) and G(h2) is mapped to the correction factor of the entire region of the taken image, and the taken image is corrected.
Japanese Laid-open Patent Publication No. 2008-79248 discloses that while the correction factor of the overlap region R is calculated by the same method as the technique disclosed in the Japanese Laid-open Patent Publication No. 2008-77595, the correction factor at the central position of the image taking region E is determined as 1. The correction factor of the region between the overlap region R and the center position of the image taking region E is interpolated based on the ratio of the distance between the overlap regions.
Specifically, as illustrated in FIG. 19, a pixel S exists in the image taking region E4 of the camera C4 other than the overlap region R. When the length of a line drawn from the pixel S onto the optical axis of the camera C is Lx, the length from the pixel S to the overlap region R1 is Ly, and the correction factor in the overlap region R1 is G, the correction factor G(S) in the pixel S is calculated by G(S)=1+(G−1)(Lx/(Lx+Ly)).
When the image taken by the camera C4 is corrected, for example, the correction factor of the overlap region R1 is determined as G(S1), and the correction factor of the overlap region R4 is determined as G(S2). As shown in FIG. 21, the correction factor from the left end of the image taking region E4 to a pixel S1 is G(S1) and constant, and the correction factor from the right end of the image taking region E4 to a pixel S2 is G(S2) and constant. The correction factor of a pixel S0 on the optical axis of the camera C4 is 1, and the correction factor between the pixel S1 and the pixel S0 and the correction factor between the pixel S2 and the pixel S0 are calculated based on the above formula.